1. Field of the Invention
The present invention relates to an apparatus for forming a multi-layer parison having main and auxiliary resin layers laminated with each other by means of extrusion molding in which the multi-layer parison is introduced into a cavity of a molding die.
2. Description of the Related Art
An apparatus for forming a multi-layer parison by means of extrusion molding comprises an accumulator for extruding a main resin by applying a pressure thereto and accumulators each for extruding an auxiliary resin by applying a pressure thereto. The main and auxiliary resins extruded from nozzles of the accumulators are laminated with each other in predetermined thicknesses. Thus, the multi-layer parison is formed.
The multi-layer parison formed in this way is introduced into a cavity of a molding die. Thereafter, air is blown thereinto and the multi-layer parison is cooled in close contact with the inner face of the cavity. Such process is called blow molding, whereby a hollow body, for example, a multi-layer container is molded.
In the aforementioned apparatus, the extrusion of the resins from the accumulators is stopped while the accumulators are refilled with resins. When the blow molding is carried out with the use of a mold in succession to the formation of the multi-layer parison, it is necessary to stop the extrusion of the resins from the accumulators at least when the blow molding is being carried out.
When the extrusion of the resins is resumed, the auxiliary resins occasionally fail to join with the last auxiliary resin extruded in the previous process. In the case where an auxiliary resin is used as a barrier material to improve the impermeability of a multi-layer container so that liquid accommodated in the container is prevented from permeating through the container, it is necessary to perform the blow molding after removing portions having no auxiliary resin. In other words, such portions cannot be effectively used and the so-called yield gets worse.
The Japanese Patent Publication No. 58-23212 discloses an apparatus for forming a multi-layer parison, which is provided with a control device for controlling pressures for extruding main and auxiliary resins so that the pressures are gradually increased at the beginning of extrusion of respective resins.
According to this prior art, each resin layer is prevented from being irregularly formed by gradually increasing the extrusion pressure applied thereto so that discontinuousness of the auxiliary resins will be prevented.
Absorption and adhesion act between molten main and auxiliary resins. Accordingly, the auxiliary resins disposed in the vicinity of nozzle outlets are liable to be drawn by the main resin when the pressure for extruding the main resin increases before the pressures for extruding the auxiliary resins have not increased enough to extrude the auxiliary resins from respective nozzle outlets at the beginning of extrusion of both resins.
In particular, when a multi-layer parison consisting of two main resin layers and an auxiliary resin layer or layers interposed between the main resin layers is formed, the auxiliary resins are still liable to be drawn by the main resin because the auxiliary resins disposed in the nozzle outlets are sandwiched between the two main resins.
In the known multi-layer parison forming apparatus, pressures for extruding main and auxiliary resins gradually increase when the extrusion thereof is started. Accordingly, it is impossible to prevent the auxiliary resins from being drawn by the main resin.